In view of the ever more stringent environmental regulations, the requirements relating to the desulfurization of flue gas have become ever more exacting, in particular regarding the combustion of coal in power plants. Numerous processes are known where sulfur is to be bound by employing limestone and similar materials (German laid-open application Nos. 27 08 984 and 27 30 707). The flue-gas gypsum produced with these prior methods, however, cannot be stored nor be further used in the form obtained. Therefore, it is transformed by heat input into the so-called gypsum hemihydrate. This material can be employed for various purposes in the construction industry as well as, under certain conditions, for dam building in underground mining (German printed specification No. 29 08 266). The large consumption of energy for the conventional transformation of the dihydrate into the hemihydrate is disadvantageous. This holds also for the process known from German Pat. No. 12 38 374 according to which an aqueous suspension of calcium sulfate dihydrate is heated at a predetermined rate to a high final temperature. A disadvantageous aspect of this known process resides in the need for seed crystals and the employment of an autoclave.
Considerable amounts of waste heat are generated in power plants, especially in those where the resulting flue-gas gypsum cannot be utilized without additional treatment, this heat being uselessly transferred to the atmosphere. In addition to power plants, the chemical industry, steel plants and other branches of industry also give rise to a substantial amount of waste heat which is utilized only insufficiently or not at all.
The waste heat referred to is produced at about 130.degree. C. in such power plants and could go as high as 140.degree. C. in older installations. It is disadvantageous that this heat, on account of its relatively low temperatures and its character, cannot be used in the order of magnitude in which it is generated.